Although eradicated from Europe, bovine leukemia virus (BLV) is responsible for important economic losses worldwide. The great majority of BLV-infected animals are asymptomatic carriers of the virus. Approximately one-third of BLV-infected bovines develop a benign polyclonal proliferation of B cells called persistent lymphocytosis (PL), characterized by an increase in the absolute number of peripheral blood circulating B lymphocytes associated with an inversion of the B/T lymphocyte ratio. PL is usually stable for several years but can also progress to a tumour phase.
The most conspicuous clinical manifestation of BLV infection is the development of lymphoid tumours. Fatal lymphoma or lymphosarcoma (LS), characterized by mono- or oligo-clonal B cell expansion, occurs in less than 5-10% of infected animals, predominantly in adult cattle older than 4-5 years old. Local proliferation of B cells, called lymphosarcoma, can occur within different organs and tissues leading to a series of defects that are finally incompatible with the survival of the animal. In addition, transformed B cells can also induce the enlargement of lymph nodes and cause lymphoma. Besides an impact on survival, BLV infection also impairs the immune system leading to opportunistic infections.
Several attempts have been undertaken to develop vaccines against BLV, such as vaccines based on chemically inactivated BLV, vaccines based on lysates from, e.g., BLV-infected cells or BLV tumours, vaccines comprising BLV subunits, such as, e.g., the gp51 glycoprotein. Other attempts used vaccinia virus as a vehicle and introduced BLV genes encoding, e.g., BLV envelope proteins, into its genome (i.e., recombinant vaccinia virus or RVV). Short peptides mimicking B and T cell epitopes of BLV proteins were also tested as immunogens. DNA vaccines comprising BLV genes, e.g., the env gene under the control of the cytomegalovirus promoter, were also developed. These ‘traditional’ vaccine candidates faced problems of inter alia efficacy (i.e., only an inadequately low fraction of vaccinated animals were protected), persistence (i.e., rapid decrease of immune protection in the vaccinated animals), cost (e.g., high cost of production of purified proteins), and/or safety (e.g., use of genetically modified hybrid viruses, such as RVV).
In an attempt to address the shortcomings of these earlier approaches, numerous attenuated BLV mutants were developed, e.g., by deleting genes dispensable for infectivity but required for efficient replication of the virus (Willems et al. 1993. J. Virol. 67: 4078-4085). Among these, an attenuated BLV provirus, pBLV6073, was obtained by introducing a mutation to an immunoreceptor tyrosine-based activation motif localized in the cytoplasmic tail of the transmembrane gp30 envelope glycoprotein (Willems et al. 1995. J. Virol. 69: 4137-4141). Another attenuated BLV provirus, pBLVDX, was constructed by deleting the R3 and G4 sequences (Willems et al. 1993. J. Virol. 67: 4078-4085). These BLV mutants (pBLV6073 and pBLVDX) were evaluated in Kerkhofs et al. 2000. J. Gen. Virol. 81: 957-963; Reichert et al. 2000. J. Gen. Virol. 81: 965-969; and Florins et al. 2007. J. Virol. 81: 10195-10200.